The invention relates to an image pick-up system, comprising a television pick-up tube and a deflection voltage generator which is connected to the television pick-up tube, the television pick-up tube comprising an electron source for emitting an electron beam to an image pick-up face, a horizontal deflection device for deflecting the electron beam across the image pick-up face along an image line, and a vertical deflection device for deflecting the electron beam in a direction transversely of the image line, the vertical deflection device comprising a first deflection element and a second deflection element which is of an electromagnetic type and which operates at a frequency which is higher than that of the first deflection element.
An image pick-up system of this kind is described in the not previously published Netherlands Patent Application No. 9001171, corresponding to U.S. patent application Ser. No. 699,853 filed on May 14, 1991, in the name of Applicant. The cited Patent Application describes a television pick-up tube which comprises deflection coils for deflecting the electron beam across an image pick-up face in two mutually perpendicular directions. The image pick-up face is scanned, for example line-wise, an image being composed of two interlaced fields comprising 312.5 lines each. For deflection of the electron beam the deflection coils receive a sawtooth voltage, a frequency of the sawtooth voltage across the coil for deflection in the vertical direction amounting to, for example 50 Hz while a frequency of the sawtooth voltage across the coil for deflection in the horizontal direction amounts to 15.6 kHz.
When use is made of a television pick-up tube whose image pick-up face is large in comparison with the beam diameter of the electron beam, for example as in medical imaging systems comprising a chain including an X-ray image intensifier tube and a television pick-up tube, charge is liable to remain between the image lines on the image pick-up face, which charge is not compensated for by the electron beam. A positive charge accumulation attracts the electron beam in an undesirable manner so that an annoying local flicker effect occurs in the television image when the image is displayed on a television monitor. In order to counteract this effect, the vertical deflection device comprises a second deflection element which is of an electromagnetic type and which operates at deflection frequencies which are higher than those of the first deflection element of the vertical deflection device. The second deflection element periodically deflects the electron beam between transverse positions, situated to both sides of an image line, during deflection of the electron beam along the image line. The electron beam follows an undulating path around an image line across the image pick-up face under the influence of a deflection voltage of comparatively high frequency (for example, 50 MHz) applied to the second deflection element. Thus, the charge accumulated on the image pick-up face between two neighbouring image lines is compensated for and negative effects due to charge accumulation are counteracted without affecting the image resolution to a significant extent.
The video signal generated by the television pick-up tube has a frequency in the order of magnitude of 9 MHz in the case of display of images which are composed of, for example two interlaced fields comprising 312.5 image lines each with a frequency of 25 Hz (field frequency is 50 Hz). For high-resolution images (double the number of lines), the video frequency is 18 MHz. These video signals are amplified by means of a video amplifier which has a cutoff frequency beyond 18 MHz.
The second deflection element, being formed, for example by a saddle-shaped coil, can constitute, by way of capacitive coupling to the environment, a resonant circuit which has a comparatively low resonance frequency and below this resonance frequency it is liable to interfere with the video signal generated by the television pick-up tube. This occurs inter alia when signals of the deflection element are amplified by the video amplifier. Beyond the resonance frequency the behaviour of the second deflection element is capacitive, so that the deflection frequency cannot exceed the resonance frequency.